1. Field of the Invention
This invention relates to improved devices for filtration of fluid suspensions that form flow resistant filter cakes. More particularly, it concerns (a) devices for the filtering of wastewater treatment sludges or similar material that is prone to form filter cake very resistant to flow of filtrate through it and (b) methods for filtering such material at a relatively rapid rate to obtain truckable cake of relatively high solids content.
2. Description of the Prior Art
There are a variety of liquid suspensions encountered today in large amounts that must have liquid removed therefrom inexpensively to produce a filtrand of substantially increased solids content that may be handled, such as by trucking, to disposal. For example, the treatment of municipal wastewater produces large amounts of sludges that present a myriad of problems in handling and disposal. Further, continued residential, commercial and industrial developments require the provision of ever increasing wastewater treatment capacity, resulting in a corresponding increase in plant effluent and by-products. The disposal of the resulting sludge and related materials becomes ever more serious, not only because of the increase in their total volume, but also because of the increase in disposal restrictions imposed by environmental regulations.
While the invention is described with particular reference to wastewater sludge dewatering, the new devices and methods may be effectively used for the filtration of a wide spectrum of other fluid suspensions of particulate material.
In the operation of sewage treatment plants and the process of dewatering sewage sludge as a step in disposing of it, the sludge influent to the dewatering step will contain about 0.5% to 5% solids. In order to handle the sludge as a "dry product" for trucking to a disposal site, the sludge should contain at least about 10% solids.
Wastewater treatment sludges can be divided into a few basic types, e.g., alum sludge (chemical), hard sludge, aerobic domestic sewage sludge (bio-mass) and anerobic digested domestic sewer sludge (bio-mass). The present invention relates to new methods capable of handling all of such classes of sludge.
Typical filter apparatus for sludge dewatering is an open tank containing a bed of loose sand supported on a lower bed of gravel which, in turn, is supported upon the floor of the tank, usually made of concrete. This type equipment is referred to in the trade as GFM equipment, i.e., granular filter media equipment (see for example U.S. Pat. No. 1,802,726).
As water is removed from the influent in such equipment a filter cake builds up on the top of the sand layer until the cake contains about 6% solids. Depending upon the amount of time the sludge charge is allowed to remain in the equipment, it can be dewatered up to about 12-45% solids. The sludge is removed from the sand bed by shovels, back-hoes or other scraping or scooping devices. Usually a thin layer of sludge will remain on the top of the sand bed and will hinder the dewatering of the next batch of influent. Rakes or scrapers are often used to remove this sludge residue, but this can result in loss of sand requiring replenishment of the sand bed. Furthermore, this GFM type sludge dewatering is undesirably slow and requires large area filters to attain needed throughput. Land acquisition costs, therefore, can be high.
Because of the problems associated with GFM dewatering of sewage sludge, much work has been done on the development of other types of dewatering methods and equipment. Another type of sludge dewatering apparatus uses a rigid bed or plate of solid particles bound in a matrix of cured resin in place of the conventional sand bed. This type of equipment is referred to as RFM equipment, i.e., rigid filter media equipment and examples of RFM devices are disclosed in the following U.S. Pats. Nos. 4,208,288, 4,309,292, 4,340,478, 4,382,863, 4,381,998, 4,399,042, 4,431,549, 4,452,698, 4,481,114.
Still other developments in sludge dewatering have included (a) vacuum assisted rotating filters (see U.S. Pat. No. 2,230,385; (b) rotary vacuum drums (see U.S. Pat. No. 3,338,412); (c) pressurized vessels (see U.S. Pat. No. 3,397,140) and (d) vacuum assisted horizontal moving belt filters (see U.S. Pat. No. 4,217,219).
The use of vacuum to improve the efficiency of filtering operations was well known much prior to the vacuum filtering operations mentioned above with regard to sludge dewatering (see U.S. Pat. Nos. 809,861; 1,135,080 and 1,595,344). Also the concept of vacuum assist has been used in the dewatering of other sludges than sewage sludge, e.g., sludges generated in the metal, mining and chemical industries (see U.S. Pat. Nos. 3,870,641 and 3,929,642).
In addition to horizontal, rotary filter equipment mentioned above, numerous other devices of this general type have been developed including some with external or internal longitudinal spray units, hot air applicator and slotted screen drums as shown by:
U.S. Pat. Nos. 983,735, 2,257,686, 3,117,927, 3,979,289, 4,198,299, 4,248,709, 4,306,975, 4,439,320.
The present invention provides new improvements in filtering and dewatering equipment that comprise rotary filter equipment in unique combinations with other essential units and in filtering methods that utilize such equipment.